Transformer compensated for precise measurements



May 27, 1941. w. M. LEE 2,243,162

TRANSFORMER COMPENSATED FOR PRECISE MEASUREMENTS Filed April 12, 1938 near rue: :g/

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INVENTOR Mum/7 m 65 w/uey V0471 BY 3 g z ly 2 ATTORNEY/ Patented May 27, 1941 TRANSFORMER COMPENSATED FOR PRE- CISE MEASUREMENTS William M. Inc, Covlngton, Ky., aasignor to The Kelley-Koett Manufacturing Company, Incorporated, Covington, Ky., a corporation of Ohio Application April 12, 1938, SerialNo. 201,592

4 Claims. (ci. 171-95) This invention relates to a voltmeter circuit particularly intended for use with x-ray apparatus, altho it may be applied generally.

Among the objects of the invention .is the provision of means for presetting, accurately and made to indicate a true voltage of the secondary circuit regardless of the load put upon the transformer.

Previous to this invention it has been the custom in the X-ray art to estimate the secondary output voltage of the high tension transformer by a voltmeter inserted in the primary. This voltage was read by multiplying the primary voltage by a factor. Since this factor varies with the load applied, accurate readings were not obtained.

The apparatus. of the present invention oper-. ates on the principle of opposed (bucking) voltages between the main winding of an autotransi'ormer and a winding auxiliary thereto or between an auto-transformer and an entirely separate auxiliary transformer. The auto-transformers purpose is of course the conventional one of supplying current at conveniently selected voltages to the primary of the high tension transformer. The bucking voltages are applied to the voltmeter at 180 phase difference from each other so that the resultant reading is a voltage compensated for the drop to be occasioned when a greater load is thrown upon the high tension transformer. H While the compensation is automatic as to the reading it is not automatic to the extent of changing the transformer setting to increase the voltage. This operation must be done manually.

In the drawing, Fig. l shows a wiring diagram in which the bucking voltage is supplied by an auxiliary winding on the same core as the auto transformer. Fig. 2 shows the same diagram except that a the same, "t3 the primary. In former practice a voltmeter ll wlasconnected immediately across the primary leads. Here it is so connected thru an auxiliary winding I! (Fig. l) of the autotransformer I. or thru an auxiliary transformer I! (Figure 2) the primary ll of which is connected directly across the auto-transformer leads lO-ll. The secondary is connected from the high tension primary input leads to a variable resistance 20 connected across the auxiliary transformer secondary 2 I.

In Fig. 1 the variable resistance 2| is also present and connected across the auxiliary winding ll. The latter and the auto-transformer I! are wound upon the. same core 22 but the leads 22 of winding i5 are'reversed so as to make their voltage buck that of the auto-transformer It. The effect is produced also by the same reversal of leads in the case of the auxiliary transformer. The degree of phase diiierence produced is 180. In Fig. 2 the auxiliary transformer I1 is arranged to give an effect similar to that of winding [5. In operation, the switch 25 leading to the service mains is first closed and the filament of the X-ray tube is lighted by means of the convenseparate auxiliary transformer is provided as a source of the opposing voltage.

Fig. 3 is a graph showing how the voltage of In Figs. 1 and 2, i0 is an x-ray tube, II a high tension transformer, l2 the secondary of tional heating transformer not here shown. The slider 21 is then moved to the position as indicated It must be kept in mindthaheachtime a different load is desired, slider 21 must be moved to a position which has previously been determined for this particular load. I

It will be apparent that the only difference between Figs. 1 and 2 is that in the formler the bucking coil is wound upon the samecore as the auto-transformer while in Fig, 2 it is wound upon a separate 001 e. The principles and operation are identical.

The graph which forms the subject matter of Fig. 3, illustrates what occurs during operation. The kv. of output of the high tension transformer secondary are indicated vertically and the load voltage of .the high tension primary are laid oi! horizontally. As the load on the secondary is increased, it is necessary to apply more voltage to the primary in order to keep the voltage of the.

secondary constant. The increasing load in current (milliamperes) is shown by the oblique lines. The horizontal dotted line is the output'kilovoltagethahone desires to maintain. The ertical lines indicate the increased amount of volt- It is to be recognized that a structure in which the high-tension transformer in my invention is replaced by a step-down transformer for use in welding or the like, is the full equivalent of th step-up circuits herein shown.

The value of the resistance 20 is chosen preferably so that the amount of current drawn from the auxiliary transformer or winding will not be excessive, but otherwise the value in ohms is not important. a

From the foregoing, it will be evident to skilled electrical engineers that a method of general application has been devised by which 9. voltmeter may be compensated. It is independent of the particular circuits shown and comprises the following steps:

1. Calibrate high tension transformer at no load, primary against secondary voltage.

2. Apply load to transformer and calibrate the transformer in terms of the primary voltage for that load.

3. Select a point by trial on resistance connected across a voltage bucking coil that will make voltmeter indicate the proper voltage.

4, Apply load and adjust auto-transformer taps until voltmeter indicates desired voltage for that load.

-5. In like manner calibrate for any other load.

I claim as my invention:

1. In a device of the character described, an auto-transformer having a plurality of taps, a high tension transformer, the primary of which is connected to the taps of said auto-transformer, a core for said auto-transformer, an auxfliary winding upon said core adapted to oppose the voltage generated by said auto-transformer at substantially a'variable resistance connected across said auxiliary coil, the resistance and auxiliary winding being connected across the connections between the auto-transformer and the high tension transformer, and a voltmeter in series with the resistance and auxiliary winding.

2. In a device of the character described, an

auto-transformer having a plurality of taps, a

high tension transformer the primary of which is connected to the said taps, an auxiliary transformer connected. to said auto-transformer and adapted to oppose the voltage of the latter at substantially 180, a secondary for said auxiliary transformer, a variable resistance connected across said secondary and across the connections between the auto-transformer and the high tension transformer and a voltmeter in series with said resistance.

3. In a device of the character described, an auto-transformer, a high tension transformer connected to said auto-transformer, means for varying by steps the voltage fed to said high tension transformer, a voltmeter connected to the leads of said auto transformer and the primary of said high tension transformer, an auxiliary transformer bucking the auto-transformer voltage, in series with-said voltmeter and autotransformer and a variable resistance connected across said auxiliary transformer.

4. X-ray control apparatus of the character described, comprising a transformer adapted for connection to a line and provided with means for varying its output voltage, a high tension transformer energized thereby and adapted to supply current for the X-ray tube, an indicator for said output voltage, and adjustable compensating means in circuit with said voltage indicator, said line transformer being provided with means for supplying voltage to said compensating means independently of the high tension transformer connections to said line transformer, and said compensating means being adapted to insure accuracy of said voltage indicator for each position of said compensating means.

WILLIAM M. LEE. 

